1. Field of the Invention
The present invention relates to a miniature fluxgate sensor that is manufactured via a thin-film process, and to an electronic compass in which this fluxgate sensor is used. In particular, the present invention relates to a fluxgate sensor that not only is small in size and is highly sensitive, but also has a high level of excitation efficiency and a high degree of design freedom, and to an electronic compass in which this fluxgate sensor is used.
2. Description of the Related Art
Conventional magnetic sensors include those that utilize the Hall effect, those that utilize a magnetoresistive (MR) effect, and those that utilize a giant magnetoresistive (GMR) effect. Because these are manufactured via a thin-film process, they can be miniaturized and integrated, and are therefore widely used in portable devices and the like.
However, the sensitivity of these sensors is lowered when they are miniaturized, and it becomes difficult for geomagnetic levels of approximately 0.3 Oe, such as are detected by an electronic compass, to be detected accurately, and the azimuth accuracy in electronic compasses that use these sensors is limited to approximately 10 degrees.
Moreover, in recent years, electronic compasses that are based on magneto-impedance sensors (referred to below as MI sensors) that use amorphous wire, and orthogonal fluxgate sensors have been proposed, and highly accurate compasses that provide an azimuth accuracy of approximately 2.5 degrees have been achieved. In addition, electronic compasses that use miniature fluxgate sensors that are manufactured via a thin-film process have also been proposed (see, for example, Japanese Unexamined Patent Applications, First Publications Nos. 2007-279029, 2006-234615 and 2004-184098, and PCT International Publication No. WO 2007-126164 Pamphlet).
In order to increase azimuth accuracy, in particular, the detection resolution and linearity errors that are determined by the sensor sensitivity are important elements. In MI sensors, orthogonal fluxgate sensors, and fluxgate sensors, the resolution is regarded as being approximately the same. Moreover, a large number of components such as speakers, vibration motors, and magnets and the like that serve as magnetic field generation sources are mounted inside the devices, and the sensors are affected by the magnetic fields generated from these components. In order for a sensor to operate correctly in the presence of a magnetic field generated from a peripherally placed component, it is desirable for the measurement magnetic field range to be sufficiently broad.
If linearity errors are considered, then in the case of MI sensors and orthogonal fluxgate sensors, hysteresis is also output in the output voltage due to hysteresis in the magnetic core. Because of this, linearity errors are made worse. In order to improve linearity, a method that utilizes a negative feedback circuit may be used, however, this causes power consumption to increase and also makes the circuit more complex.
In contrast, in the case of a fluxgate sensor, by using the phase-delay method described in Pavel Ripka, “Magnetic Sensors and Magnetometers”, p. 94, ARTECH HOUSE, INC (2001), a magnetic sensor that is not affected by magnetic core hysteresis and has superior linearity can be achieved. If this method is used, the sensor output is made based on a time domain, and not only is it possible to remove the effects of hysteresis which is caused by the coercive force of the magnetic core that makes up the sensor, but it is also possible for digital detection using a counter to be made. As a consequence, it is possible to remove the effects of errors which occur during A/D conversion, and it is possible to construct a sensor having superior linearity.
According to IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 42, NO. 2, p. 635, APRIL 1993, by using the aforementioned method, a linearity of 0.06% FS can be achieved. In an MI sensor that uses amorphous wire, linearity errors are 1-2%. Consequently, by using a fluxgate sensor having superior linearity in this way, it is possible to achieve an electronic compass having greater azimuth accuracy.
As is described above, by using a fluxgate sensor that employs a phase-delay method which has a high resolution and superior linearity, it is possible to construct an electronic compass having excellent azimuth accuracy. However, it is necessary in a fluxgate sensor for an excitation coil and detection coil to be wound around the periphery of the magnetic core. Accordingly, compared with an MI sensor or an orthogonal fluxgate sensor that are constructed with only a bias coil or a pickup coil being wound around the core, it is difficult to achieve a miniaturization of the size of a fluxgate sensor.
In order to achieve further miniaturization and integration, attempts are being made to manufacture a fluxgate sensor using a thin-film process. However, the diamagnetic field effect is increased by the miniaturization so that there is a decrease in sensitivity. In particular, if this type of sensor is used to try and make an electronic compass which has sensitivity in three axial directions, then it is necessary to set a magnetic sensitivity direction in a perpendicular direction relative to the substrate forming part of the electronic compass, so that, as a result, it is necessary to package the sensors in a vertically upright state on the substrate used to form the electronic compass. As a consequence, when the aim is to make a thinner electronic compass, then it is necessary for the length of the sensor which extends vertically upright from the substrate to be shortened in the direction of the magnetic sensitivity thereof. For example, if the thickness of the electronic compass needs to be kept to 1 mm or less, then considering the thickness of the substrate and the molding resin, it is necessary to keep the length of the sensor in the direction of magnetic sensitivity to approximately 0.5 to 0.7 mm. However, if the length of the magnetic core is no more than 1 mm, the effect of the diamagnetic field is increased and there is a marked decrease in sensitivity.
In order to solve the above described problems, Japanese Unexamined Patent Application, First Publication No. 2007-279029 and PCT International Publication No. WO 2007-126164 Pamphlet disclose an H-type of magnetic core in which the width of the end portions of the magnetic core has been widened. In this structure, the excitation coil and the detection coil are only wound around the narrow portion of the magnetic core center portion. Accordingly, if the size of the sensor is reduced, the number of coils that can be wound around the excitation coil and magnetic coil is restricted, and it becomes difficult to secure a sufficient number of coils. Because the structure is one in which the excitation coil and the magnetic coil are wound alternatingly, the number of coils ends up being determined by the sensor size and the coil pitch. Accordingly, it becomes difficult to set the number of coils of both the detection coil and the pickup coil independently of each other, so that the degree of design freedom is constricted.
FIG. 15 is a schematic view showing the shape of a magnetic core of a conventional fluxgate sensor. The magnetic core has end portions 1 and a center portion 2. In Japanese Unexamined Patent Application, First Publication No. 2007-279029, it is disclosed that a ratio B/A of the width B of the end portions 1 shown in FIG. 15 relative to the length A of the magnetic core in the longitudinal direction is preferably 0.8 to 1.2. It is also disclosed that a ratio C/B of the width C of the center portion 2 shown in FIG. 15 relative to the width B of the end portions 1 is preferably 0.033 to 0.2. When the value of the ratio B/D of the width B of the end portions 1 shown in FIG. 15 relative to the length D of the end portions 1 in the longitudinal direction exceeds 1, then the length of the magnetic core in an orthogonal direction relative to the magnetic sensitivity direction of the sensor becomes longer. Accordingly, the shape magnetic anisotropy in the end portions 1 has an easy axis in the transverse direction of the sensor. As a consequence, the flux density of the end portions 1 tends to become more sensitive to the magnetic field in an orthogonal direction relative to the magnetic sensitivity direction of the sensor. As a result, if an electronic compass is created by arranging a plurality of the above described fluxgate sensors perpendicularly to each other, the magnetic cores of the fluxgate sensors are easily affected by the magnetic field in an orthogonal direction relative to the detection magnetic field, and the cross-axis sensitivity of the electronic compass increases. Moreover, because a distortion is generated in the pickup waveform by the magnetic field in an orthogonal direction relative to the detection magnetic field, output abnormalities are easily generated and the orthogonality of each axis deteriorates. Here, the term cross-axis sensitivity refers to changes in the output of the magnetic field in the X-axial direction in a sensor whose magnetic sensitivity direction is the Y-axial direction or the Z-axial direction when, for example, the magnetic field in the X-axial direction is being detected. If the cross-axis sensitivity increases, the orthogonality of the axes deteriorates, and the azimuth accuracy of the electronic compass also deteriorates. In addition, not only are pulse-shaped temporal changes in the pickup voltage included in the cross-axis sensitivity, but changes in output that are caused by changes in the pulse waveform itself are also included therein.